In certain refrigeration and air conditioning applications, a heat exchange unit has a liquid refrigerant flowing within a tube while the fluid to be cooled flows externally over the tube. Liquid refrigerants such as trichloromonofluoromethane or dichlorodifluoromethane flow through the tube. As the liquid refrigerant absorbs heat from the external liquid, the refrigerant is changed to a gas. The gas phase refrigerant is returned to a compressor, compressed to a liquid and returned to the heat exchange tube for another cycle.
One method to form the tubes involves passing a metallic strip through forming rolls to transform the strip into an ellipsoid with longitudinal edges adjacent one another. The edges are then welded together to form a tube. This process is disclosed in U.S. Pat. No. 4,995,549 to Hellman, Sr., which is incorporated by reference in its entirety herein.
To increase the efficiency of heat transfer through the tube, the inner or outer surface of the tube may be enhanced. Enhancements consist of fins, protrusions or other shapes which increase the surface area. A plurality of parallel fins is disclosed in U.S. Pat. No. 4,658,892 to Shinohara et al. while truncated pyramids are disclosed in U.S. Pat. No. 5,070,937 to Mougin et al., both of which are incorporated by reference in their entirety herein.
Another method to increase the heat transfer is by facilitating nucleate boiling. As the refrigerant changes state from a liquid to a vapor, a large quantity of heat is absorbed from the fluid. In nucleate boiling, liquid adjacent to a trapped vapor bubble is super heated by the heat exchange surface. Heat is transferred to the bubble at the liquid vapor interface. The bubble grows in size until surface tension forces are overcome by buoyancy and the bubble breaks free from the surface. As the bubble leaves the surface, fresh liquid wets the now vacated area. The remaining liquid absorbs heat from the tube surface to form the next bubble. The vaporization of liquid and continuous stripping of the heated liquid adjacent to the heat transfer surface, together with the convection effect due to the agitation of the liquid pool by the bubbles, results in an improved heat transfer rate for the heat exchange surface.
One effective nucleate boiling site is a channel adjacent to a surface of the heat exchange tube for transport of the liquid. This channel has narrow openings through which the vapor bubbles escape. The openings are sufficiently small to effectively retain the trapped vapor bubbles until super heated.
The manufacture of nucleate boiling sites is disclosed in U.S. Pat. Nos. 3,696,861 to Webb and 4,059,147 to Thorne. Fins are formed on a heat exchange surface and then bent such that the tip of one fin is in close proximity to a mid-point of an adjacent fin. A channel is formed at the base of the fins and a narrow aperture sufficiently small to promote and sustain nucleate boiling of a fluid forms where a fin tip abuts an adjoining fin. Both the Webb and the Thorne patents are incorporated in their entirety by reference herein.
One method of enhancing a tube is to emboss a desired pattern into the metallic strip prior to forming the strip into a tube. The longitudinal edges of the enhanced strip are then welded together. U.S. Pat. Nos. 3,861,462 and 3,902,552, both to McLain and both incorporated by reference in their entireties herein, disclose the use of textured rolls to emboss a pattern into the metallic strip. A desired texture may be formed on one or both sides of the strip.
Whether the fins are formed by rollers embossing the outside of a tube or textured rolls embossing a strip which is subsequently formed into a tube, the fins are tapered. The fins are thicker at the fin root than at the fin tip. Also, the merge between the fin root and the outside wall of the tube is at a substantial radius. Both the taper and the radius strengthen the fin root. As a result, when the fin is bent to form a nucleate boiling surface, the bend is about a mid-portion of the fin rather than at the fin root.
A problem with present heat exchange tubes are the difficulty with providing an accurate bend in all fins having the same arc and the same radius so that the heat transfer coefficient of the tube is predictable and repeatable. Another problem is that the super heated vapor bubbles are pressurized and interfere with the flow of fluid through the channels reducing the heat transport capability.